The organic electroluminescent device is a self-emitting device, and has been actively studied for their brighter, superior viewability and the ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang and colleagues at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic electroluminescent device with organic materials. These researchers laminated an electron-transporting phosphor and a hole-transporting organic material, and injected the both charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see, for example, Patent Documents 1 and 2).
To date, various improvements have been made for practical applications of the organic electroluminescent device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescence device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate (see, for example, Non-Patent Document 1).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and use of phosphorescent materials have been investigated (see, for example, Non-Patent Document 2).
The light emitting layer can be also fabricated by doping a charge-transporting compound, generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing lecture preprints, selection of organic materials in an organic electroluminescent device greatly influences various device characteristics, including efficiency and durability.
In an organic electroluminescent device, the charges injected from the both electrodes recombine at the light emitting layer to cause emission. However, because the hole mobility is faster than the electron mobility, some of the holes pass through the light emitting layer. This is problematic as it lowers efficiency. Accordingly, there is a need for an electron transport material with fast electron mobility.
Tris(8-hydroxyquinoline)aluminum (hereinafter, “Alq3”), a representative light-emitting material, is generally used also as an electron transport material. However, with a work function of 5.8 eV, the material cannot be said as having hole blocking performance.
Insertion of a hole blocking layer is one method of preventing the passage of some of the holes through the light emitting layer and improving the probability of charge recombination at the light emitting layer. Examples of the hole blocking materials proposed so far include triazole derivatives (for example, see Patent Document 3), bathocuproin (hereinafter, “BCP”), and a mixed ligand complex of aluminum (BAlq) (see, for example, Non-Patent Document 2).
On the other hand, 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter, “TAZ”) is proposed as an electron transport material having excellent hole blocking performance (see, for example, Patent Document 3).
TAZ has a large work function of 6.6 eV and a high hole blocking ability, and is thus used as an electron-transporting hole blocking layer laminated on the cathode side of a fluorescent layer or a phosphorescent layer produced by methods such as vacuum vapor deposition and coating. TAZ thus contributes to improving the efficiency of an organic electroluminescent device (see, for example, Non-Patent Document 3).
One big problem of TAZ, however, is the poor electron transportability, and the material is required to be combined with an electron transport material having higher electron transportability for the production of an organic electroluminescent device (see, for example, Non-Patent Document 4).
BCP has a large work function of 6.7 eV and a high hole blocking ability. However, the low glass transition point (Tg) of 83° C. makes the thin film stability poor, and the material cannot be said as being sufficiently functional as a hole blocking layer.
Either of the materials lacks film stability, or does not sufficiently serve to block the holes. In order to improve the characteristics of an organic electroluminescent device, there is a need for an organic compound that exhibits excellent electron-injecting/transporting performance with high hole blocking ability, and has high stability in the thin-film state.